In an air bag system of a vehicle, an acceleration sensor (also abbreviated to “sensor” sometimes from now on) is used for collision detection, and generally, the acceleration sensor for an air bag has a self-diagnostic function, which decides the presence or absence of a fault by electrically driving the acceleration sensor at power-on and by obtaining an output response thereto.
As for the self-diagnostic technique of the sensor, there are many patent applications conventionally. For example, the following are known: an acceleration sensor that conducts a diagnosis whether the sensor is operating normally by carrying out pulsatile self-driving and by making a decision as to whether the sensor output during the driving is contained within a prescribed threshold (see Patent Document 1, for example); an impact detecting device that carries out a self-diagnosis of an impact detecting system including a filter circuit using the self-diagnostic function of an acceleration sensor (see Patent Document 2, for example); and a sensor system that makes a self-diagnosis by comparing two sensors (see Patent Document 3, for example).
Patent Document 1: Japanese Patent Laid-Open No. 5-322925/1993
Patent Document 2: Japanese Patent Laid-Open No. 11-83891/1999
Patent Document 3: Japanese Patent Laid-Open No. 2005-283486.
However, according to the technique disclosed in the foregoing Patent Document 1, if an input affecting the sensing is applied from outside during the self-diagnosis, it cannot obtain a normal diagnostic result because the sensing element responds to both the self-driving and external input. For example, even if the sensor is normal, if the sensor output exceeds a prescribed threshold because of an external impact applied during the self-diagnosis, it makes an erroneous decision of being abnormal.
In addition, according to the technique disclosed in Patent Document 2, although the foregoing erroneous decision can be avoided because the filter circuit is also diagnosed besides the acceleration sensor, it does not set the filter circuit for diagnosis. Furthermore, according to the technique disclosed in Patent Document 3, when it conducts the self-diagnosis by comparing the two sensors, it has a problem of being unable to detect an abnormality occurring in an offset circuit such as a power circuit because it employs relative comparison.
According to the conventional techniques including the techniques disclosed in the foregoing Patent Documents 1-3, they carry out the diagnosis by comparing the sensor output itself at the self-driving with the prescribed threshold. Accordingly, when an external force affecting the sensing is applied from outside during the self-diagnosis, the sensors cannot obtain a normal diagnostic result because they respond to both the self-driving and external force.
More specifically, even if the sensors are normal, if they exceed the prescribed threshold during the self-diagnosis because of the external force (such as an impact occurring when closing a door of a vehicle), they will make an erroneous decision of abnormality. On the contrary, even if the sensors are in an abnormal state of being unable to operate by the self-driving, if they stay within the prescribed threshold accidentally because the sensors respond to the external forces they will make an erroneous decision as being normal. It becomes a fatal problem when the sensors are applied to occupant protective apparatuses such as air bag firing devices.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a vehicle state detecting device and an occupant protective apparatus having the same device capable of conducting self-diagnosis accurately even if an unnecessary external force which occurs in a vehicle and affects the sensing, is applied.